In order to reduce undesirable emissions, a combustion engine may be provided with a catalytic converter in its exhaust conduit. The catalytic converter typically includes a substrate having a surface coated with one or more catalysts promoting chemical reactions in which predetermined undesirable combustion products are reduced in quantity. A pre-catalyst oxygen sensor provides an air/fuel ratio indicating signal to the fuel charge apparatus of the engine in a closed loop control to maintain the combustion air/fuel ratio as close to stoichiometric as possible for maximum converter efficiency in reducing undesirable emissions. But the catalytic substances are subject to possible degradation that may reduce, or in some cases destroy, their catalytic effectiveness so that the catalytic converter is not operational.
In order to detect a catalyst sufficiently degraded to be considered non-operational, a prior art method uses a second, post-catalyst oxygen sensor to provide an additional air/fuel ratio signal to the engine control which may be compared to that provided by the pre-catalyst sensor in a known manner to detect a non-operational status of the catalyst. But in engines meant for operating environments with high moisture levels, such as marine or industrial, it is difficult and/or expensive to integrate a post-catalyst oxygen sensor into the exhaust stream because water intrusion leads to oxygen sensor failure. It is an object of this invention to provide a method of detecting a non-operational catalytic converter during engine operation and providing a signal thereof in a manner without using a post-catalyst oxygen sensor.